Bainitic iron alloys



United States Patent Metal Climax, Inc., New York, N.Y., a corporationof.

New York No Drawing. Filed Feb. 24, 1965, Ser. No. 435,064 4,Claims.(Cl. 14812.3)

This application is a continuation-in-part of my copending application,Serial No. 365,815, filed May 7, 1964, now abandoned.

The present invention resides in a novel and useful structural steelcomposition characterized by its high strength and low cost and in amethod of treating the same to obtain optimum properties.

So-called structural steel, which is hot rolled in the form of I-beams,channels, angle irons, plates, rods, etc., constitutes a major portionof all steel used. It is a low carbon steel of minimum cost and haslimited yield strength compared with the more expensive alloy steelswhich are of greater strength because of their capacity to be hardenedby heating and quenching. It has been recognized for many years that thedesigns of steel structures such as buildings, bridges and the likecould be improved and a saving in material realized if strongermaterials were available. However, the added cost of all prior alloysteels has more than otfset their advantage from a strength standpointwhen used in place of conventional structural steel, except in unusualsituations.

Accordingly, it is an object of this invention to provide structuralsteel members which are aseasily welded or otherwise fabricated asconventional structural steel members but which have significantlyhigher yield strength, and which are sufliciently economical to have awide field of utiltiy in applications in which conventional structuralsteels have been used.

Another object is to provide a structural steel alloy which is capableof being age hardened at relatively low temperatures and which will,without any necessity of quenching or any other drastic heat treatment,develop a hardness and strength substantially in excess of that ofconventional structural steel.

Another object of the invention is to provide a method of producing aneconomical structural steel member of greater strength than those madeof conventional structural steels.

A further object of the invention is to provide a method of producing aneconomical structural steel member having unusually high toughness atvery low temperatures.

Other objects and advantages of the invention will become apparent fromthe following specification and appended claims.

In accordance with the present invention, the foregoing objects areachieved with a low carbon steel alloy containing as essentialingredients molybdenum, copper, boron and aluminum. Carbon is notessential and is preferably no more than 0.10 percent but may be presentin amounts up to 0.15 percent. Silicon and manganese may be employed inthe usual fashion for deoxidization and retained amounts of 0.40 to 0.70percent manganese and from 0.10 to 0.30 percent silicon are permissiblebut not essential.

The boron, molybdenum and aluminum are essential to produce an alloywhich will form a bainitic structure when air cooled in the usual mannerfrom conventional hot working temperatures. For this purpose, onlyminute amounts of boron are necessary, i.e., less than 0.008 percent;but in order to insure that this small quantity of boron is not tied upwith oxygen, it is necessary to incorporate aluminum in an amount inexcess of that required -for deoxidization. The aluminum which combines3,303,061 Patented Feb. 7, 1967" with the oxygen is acid insoluble, andany excess is acid soluble aluminum. The alloy must have at least 0.01percent acid soluble aluminum and preferably has a residual acid solublealuminum content in the range of 0.20 to 0.40 percent. When acid solublealuminum is present, then it is only necessary to have that very smallamount of boron which is required to produce the bainitic structure.This minimum amount is believed to be in the order of .002 percent. Themolybdenum content may range between a minimum of 0.35 and approximately0.75 percent. Larger quantities of molybdenum are not objectionableexcept to the extent that they unnecessarily increase the cost of theresulting alloy.

The normal amounts of sulphur and phosphorus present in conventionalsteels as impurities may also be present. For this purpose a maximum of0.04 percent each is tolerated.

A further essential element of the composition is copper which may bepresent in amounts ranging from 0.60 percent to 2.0 percent. Tofacilitate hot working, the

alloy preferably contains nickel ranging upwardly from 0 to 1.0 percentas the copper content increases from 0.60 percent to 2.00 percent.

Suitable alloys of the type mentioned comprise the following:

Constituent Example 1 Example 2 Example 3 Percent Percent Percent Carbon0.07 O. 10 0.05 Mangane 0. 60 0. 70 0. 50 Silicon"... 0. 10 0. 20 0. 15Molybdenum... 0. 52 0. 40 0. 60 ickel 0. 68 0. 20 0. Copper 1.16 0.80 l.50 Boron 0. 0023 0. 003 0. 004 Aluminum (acid soluble). 0. 23 0. 07 0 30Aluminum (acid insoluble)- 0. 04 0. 04 Sulphur 0. 02 0. 02 0.015Phosphorus 0. 01 0. 015 Iron Balance Balance Hot worked alloys of theabove type Wlll, on normal cooling, form a bainitic structure and willhave a tensile yield strength (0.20 percent offset) in the order of80,000 pounds per square inch in the hot worked condition. The yieldstrength of the hot worked structural members may be materiallyincreased by age hardening from one to four hours at temperatures in theorder of 900 to 1100 F.

Conventional hot rolling procedures may be employed and will result in astructural member having a low temperature ductility and toughness aboutequivalent to that of conventional structural steel. However, if therolling or other working procedures are completed at a temperature lowerthan is customary, a remarkable increase in low temperature toughness isrealized.

The bainitic structure of the alloys of the present invention resultsfrom their cooling from an austenitic condition following conventionalhot working operations. Accordingly, the alloy should be heated prior toworking to a temperature high enough to convert the alloy to austenite.The minimum temperature for this purpose will depend upon the carboncontent, but will ordinarily be around 1750 F. Initial working may beconducted at or above this temperature or may be delayed until the alloyhas partially cooled, but to obtain the maximum toughness at lowtemperatures, it is necessary to complete the hot working operation at atemperature of approximately 1200" F. Thus, for example, hammer forgingmay start at temperatures of 2000 F. or more, but, to realize maximumtoughness, should be continued as the piece cools so that final workingis efiected at a temperature in the range of 1100 F.-1300 F. andpreferably at about 1200 F. Similarly, the alloys may be hot rolled bythe procedures conventionally employed in forming structural steelmembers such as I-beams, channels, angle irons, plates, etc. In thiscase the initial rolling operations may be conducted at conventionaltemperatures such as 2050 F., but again maximum toughness is obtained ifthe final rolling pass is carried out at a temperature in the range of1100 F. to 1300 F. and preferably about 1200 F. i

Low temperature toughness is also enhanced by continuing the agehardening operation beyond the point of maximum yield strength. This isshown by the following table setting forth data obtained in Charpy V-Notch impact tests on specimens cut from a hot worked 1% inch square barhaving the composition of Example 1. This bar had been hammer forged intwo stages, the first starting at a temperature of about 2050 F. reducedthebar from 3% inches square to 2% inches square. The second stage,which started at about 1700 F. and continued until the temperature wasabout 1200 F. reduced the bar to 1% inches square.

CHARPY V-NOTCH IMPACT PROPERTIES Fracture Test Impact Lateral Appear-Condition Temp., Strength, Expanance,

F. ft.-1b. sion, in. Percent Fibrous Fracture Hot-Worked -25 4 0. 003 05 0.005 3 72 14 0. 016 13 125 36 0.018 42 Hot-Worked and Aged -25 30.001 0 4 hr., 900 F. 0 4 0. 002 0 72 '37 0. 027 17 125 59 0. 045 40Hot-Worked and Aged 25 0. 011 11 4 hr., 1,000 F. V 0 83 0.060 60 75 1110. 083 100 125 115 0. 082 100 Hot-Worked and Aged. -100 63 "0. 050 42 4hr., 1,050 F. -75 76 0. 057 77 50 109 0.077 82 -25 117 0.083 100 0 1200. 087 100 75 112 0.083 100 Hot-Worked and Aged -50 18 0. 019 27 4.hr.,1,100 F. --25 99 O. 064 72 0 116 0.083 89 75 126 0.090 100 Average ofthree tests.

It will be observed from the above that while the material has arelatively high transition temperature and low impact strength in thehot worked condition, aging 'four hours at 1050" F. and above sharplylowered the transition temperature and increased the impact strength.Aging treatments for four hours at lower temperatures, such as 900 or1000 F., or for shorter periods of time at higher temperatures, developmaximum yield strength but less impact strength. The material aged forfour hours at 1050 F. is far superior to conventional plain carbonstructural steels whichrhave a standard Charpy V-Notch impact strengthranging between about 0 and 10 ft. lbs. at 2S F. and a yield strength ofabout 35,000 p.s.1.

While in the above tests a four-hour aging treatment was employed,advantageous results may be obtained at substantially shorter periods oftime. The time of treatment is a matter of choice, bearing in 'mind thatwithin limits the longer the treatments and the higher the temperaturesof treatment, the greater the impact strength and the lower thetransition temperature with some sacrifice in yield strength. Thus, forexample, a specimen having the composition of Example 1 had thefollowing yield "strengths in the conditions indicated:

Lbs./sq. in. Hot-Worked 79,600 Aged 4 hrs. at 900 F. 98,400 Aged 4 hrs.at 1000" F 94,200 Aged 4 hrs. at 1050 F. 87,300 Aged 4 hrs. at 1100' F.84,900

In view of the above, it is apparent that if maximum impact strength isrequired, the best results are obtained with long aging treatments andthat for that purpose the optimum aging temperature for a four-hourtreatment is about 1050 F. As will be understood by those skilled in theart, longer treatments at lower temperatures or shorter treatments athigher temperatures will produce similar results. If maximum yieldstrength'is desired and some sacrifice of impact strength can betolerated, shorter times of treatment may be employed. Thus, treatmentsfor as short a time as fifteen minutes at 1100 F. or one hour at 900 F.will result in a marked increase in yield strength.

The effect of the final rolling temperature on low temperature toughnessis illustrated by the following table of Charpy V-Notch impactproperties of rolled bars at very low temperatures. These bars, whichhad the composition of Example 2, where first rolled from 1% inch squarebars to inch thick strips at 1750 F. and then rolled to /2 inch thick by1% inch wide strips at the final rolling temperature indicated below.After rolling the bars were aged four hours at 1050 F. and then subjectto the impact test at -50 F. and -125 F.

Impact Strength, Final Rolling tt.-lbs.

Temp, F.

50 F. l25 F.

While it will be apparent that the preferred embodiments of theinvention disclosed are well calculated to fulfill the objects abovestated, it will be appreciated that the invention is susceptible tomodification, variation and change without departing from the properscope or fair meaning of the following claims. For example, minoramounts of unspecified elements may be present so long as they do notimpair the beneficial eifects of the essential elements or change theessential character of the alloy.

What is claimed is:

1. Abainitic structural steel having from 0 to 0.15 percent carbon, from0.35 to 0.75 percent molybdenum, from 0 to 1.0 percent nickel, from 0.60to 2.00 percent copper, from 0.002 to 0.008 percent boron, from 0.01 to0.40 percent acid soluble aluminum, and the balance essentially iron.

2. An age hardened structural steel member having a bainiticmetallurgical structure and a finely divided copper-containingprecipitate dispersed through the crystal matrix and having acomposition containing from 0 to 0.15 percent carbon, from 0.35 to 0.75percent molybdenum, from 0 to 1.0 percent nickel, from 0.60 to 2.00percent copper, from 0.002 to 0.008 percent boron, from 0.01 to 0.40percent acid soluble aluminum, and the balance essentially iron.

3. An age hardened bainitic structural steel member having a bainiticmetallurgical structure and a coppercontaining precipitate dispersedthrough the crystal matrix .60 to 2.00 percent copper, from about .002'to about .008

percent boron, from .20 to .40 percent acid soluble aluminum, and thebalance essentially iron.

4. The method of making a structural steel member which consists inWorking a steel alloy containing from 0 to 0.15 percent carbon, from0.35 to 0.75 percent molybdenum, from 0 to 1.0 percent nickel, from 0.60to 2.00 percent copper, from 0.002 to 0.008 percent boron, from 0.01 to0.40 percent acid soluble aluminum, and the balance essentially iron, atleast a portion of said working being conducted after the alloy has beenheated to a temperature suflicient to convert it to an austenitestructure and the final working to form the structural member beingconducted while the alloy is at a temperature in the range of 1100 'F.to 1300 F., and age hardening References Cited by the Examiner UNITEDSTATES PATENTS 1,972,241 9/1934 Lorig 148-142 X 1,972,248 '9/193'4 Smith148142 X 10 3,132,025 5/1964 Hurley 75-124 DAVID L. RECK, PrimaryExaminer.

P. WEINSTEIN, Assistant Examiner.

4. THE METHOD OF MAKING A STRUCTURAL STEEL MEMBER WHICH CONSISTS INWORKING A STEEL ALLOY CONTAINING FROM 0 TO 0.15 PERCENT CARBON, FROM0.35 TO 0.75 PERCENT MOLYBDENUM, FROM 0 TO 1.0 PERCENT NICKEL, FROM 0.60TO 2.00 PERCENT COPPER, FROM 0.002 TO 0.008 PERCENT BORON, FROM 0.01 TO0.40 PERCENT ACID SOLUBLE ALUMINUM, AND THE BALANCE ESSENTIALLY IRON, ATLEAST A PORTION OF SAID WORKING BEING CONDUCTED AFTER THE ALLOY APORTION OF SAID WORKING BEING CONDUCTED AFTER THE ALLOY HAS BEEN HEATEDTO A TEMPERATURE SUFFICIENT TO CONVERT IT TO AN AUSTENITE STRUCTURE ANDTHE FINAL WORKING TO FORM THE STRUCTURAL MEMBER BEING CONDUCTED WHILETHE ALLOY IS AT A TEMPERATURE IN THE RANGE OF 1100*F. TO 1300*F., ANDAGE HARDENING THE STRUCTURAL MEMBER AT A TEMPERATURE IN THE RANGE OF900*F. TO 1100*F. FOR A PERIOD OF TIME SUFFICIENT TO LOWER ITSTRANSITION TEMPERATURE AND INCREASE ITS CHARPY V-NOTCH IMPACT STRENGTH.